1. Host Multicast: A Framework for Delivering Multicast to End Users
Jonghak Kim
Hyungik Oh
Huo Da
June, 2002
INFOCOM

2. Motivation
Fast increasing need for scalable and efficient group communication
Slow deployment of IP Multicast
Deployment imposes dependency on routers
Emerging End-host based Multicast
Member hosts duplicate and forward packets
Easy to deploy, but less efficient

3. Host Multicast A hybrid approach Goal: Ubiquitous Multicast
Design Requirements
Deployable on the current Internet
No support is required from OS, routers and servers
Enable multicast applications
Compatible with IP Multicast to the furthest extent
Use IP Multicast where available
Provide incentive to future deploy
Scalability and Efficiency
Multicast group should not be a limiting factor

4. Architecture To bootstrap new members
Rendezvous Point (HMRP) RP
IP Multicast Island
Designated Member (DM) DM
Normal Member
Host
Unicast Tunnel
A network of any size that supports IP Multicast, e.g. single host, Ethernet, campus network etc.
Host Group Management Protocol (HGMP) for intra-island management
Host Multicast Tree Protocol (HMTP) to build inter-island tunnels
Each member runs our daemon program at user-space

5. Host Multicast Tree Protocol (HMTP)
Build a bi-directional shared-tree connecting all islands
The tree should be congruent to physical network topology to be efficient
use member-to-member round-trip time as distance metric in current design
The tree should be robust
be able to handle node failure, dynamic join/leave etc.
I would like to introduce Host Multicast Tree Protocol. HMTP is a tree-based end-host multicast protocol. It was suggested to overcome limitations of IP Multicast Protocol such as the limitation of deployment of network domain, as well as slow delivery speed between domain and domain, so HMTP builds group-shared tree contrary to the source-specific tree of the IP Multicast Protocol, so the HMTP can automate interconnection of the IP Multicast, and it provides Multicast where IP Multicast is not available.
To reduce routing inefficiency, the tree should be congruent to the underlying network topology.
And it should be robust, because a tree is more fragile than a mesh in that a single node failure.

6. Join Group root HMRP always knows the root of the tree.
A newcomer does a depth-first search of the tree to find a close member as its parent.
Clustering nearby members makes the tree congruent to physical network topology to the first order. B C D A B C B A A C D D G
There are some mechanisms to detect node failure and loop formation.
First one is Join Group.
There is a HMRP where new members can learn about membership of the group.
Let say A through G are existing members, and H is a newcomer to join the group.
H asks the root of group to the HMRP.
HMRP replies that A is root.
Then, H sets A as its potential parent and asks A for a list of A’s children.
From A’s answer, H picks the closest one, in this case, let say it is D. Then D becomes a new potential parent.
H peaks this, and find new potential parent, F
If F rejects H’s request, H marks F as invalid and goes back up one level and resume the process
It eventually finds G. E G H F G F G F G
Where is my group? H RP
Root of your group is A

7. Tree Maintenance
Each member keeps its children list and root path up to date by exchanging REFRESH and PATH messages with neighbors.
Root sends REFRESH message to HMRP. B B B A A C C C D D
Second one is Tree Maintenance.
States in HMTP are refreshed by periodic message exchanges between neighbors.
1) Every child sends REFRESH messages to its parent.
2) The parent replies by sending back PATH messages.
3) PATH message sent by the parent contains the root path of the parent.
4) By appending itself to its parent’s root path, a member constructs its own root path.
5) Every member must maintain the freshness of its list of children and root path.
6) The root sends REFRESH messages to HMRP, so that HMRP always knows who is the current root. E F G F G F G G H RP

8. Member Leave and Partition Recovery
Parent deletes the leaving node from children list.
Direct Children repair the tree by running join procedure in the reverse order.
If root is leaving, the first node contacting HMRP is assigned as new root. B B B A A C C C D D E F F F G G G G H RP

9. Tree Improvement Periodically re-run the join procedure
To accommodate changes in network conditions and group membership
Start from a randomly picked node in the root path.
Less frequent than REFRESH and PATH messages.

10. Loop Detection and Resolution
Loop is possible:
Multiple conflicting joins happen at the same time.
Detection:
One’s root path contains itself
Resolution:
Leave the current parent and re-join the tree from the root.
Loop is rare. B A C D E F G

11. ~ The End ~